sunxi: SPL SPI: Split off SPI0 base address

So far on all supported Allwinner SoCs we find the old generation SPI
controller always at address 0x1c05000, and the new generation one at
0x1c68000. However the Allwinner R40 SoC has a new generation SPI at
the old address, and the H6 uses a completely different address.

So split off the base address from the respective SPI registers, by
changing the #defines to just contain offsets.
The base address is provided by a function, so it can easily be extended
later when support for those SoCs materialises.

This does not change the code size (since the toolchain is clever enough
to optimise this properly), also does not bring any functional change at
this point.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>